1. Field of the Invention
The invention relates to underground gasification of carbonaceous deposits. More specifically, the invention relates generally to an apparatus and method for the recovery of fuel products from subterranean deposits of carbonaceous matter using a mobile, electric device in a horizontal well-bore.
2. Background of the Invention
It is estimated that approximately 50% of existing coal is located too deep within the earth to be mined conventionally. Due to regulatory, safety, and environmental demands, some mining operators are being forced to close their mines, leading to a decrease in the productivity of traditional coal production in particular, and for coal use as an energy source in general. This has led to research on the feasibility of underground coal gasification, UCG.
Underground gasification has several inherent advantages over conventional mining, including the avoidance of safety and health hazards related to the underground mining of coal, avoidance of the environmental impact which occurs during strip mining of coal, avoidance of problems of spoil banks, slag piles and acid mine drainage. Additionally, UCG has demonstrated an ability to recover coal from seams unsuitable for conventional mining techniques.
In U.S. Pat. No. 4,776,638, Hahn describes a method for electro-thermal and electrochemical underground conversion of coal into oil and by-products. The method comprises inserting an underground probe into a bore hole until the probe is in close proximity with a coal seam. A mixture of air, steam, an electrolyte, and a suitable catalyst is supplied to the probe via a feed supply line, and the mixture is sprayed directly on the coal seam through a passage in a nozzle. Tunnels of limited horizontal reach, about 100 to 150 feet in length, are formed by advancing the probe away from the vertical well bore in a substantially horizontal direction into and through the coal seam during conversion. Products are removed from the same vertical well bore.
In U.S. Pat. No. 4,067,390, Camacho et al. describe an apparatus and method utilizing a plasma arc torch as heat source for recovering useful fuel products from in situ deposits of coal, tar sands, oil shale, and the like. When applied to a coal deposit, the plasma torch is lowered in a vertical shaft into the deposit and serves as a means for supplying heat to the coal and thereby stripping off the volatiles. The fixed carbon is gasified by reaction with steam that is sprayed into the devolatilized area and product gases are removed from the same vertical shaft. Umbilicals are provided for carrying electrical power, plasma gas, and cooling water. The plasma arc torch operates in a transferred mode wherein the arc is attached to an external forwardly-placed, axially aligned torch-mounted electrode.
In U.S. Pat. No. 4,648,450, Gash et al describe an underground coal gasification process containing a system of injection and production wells. The injection well is positioned at an angle with respect to horizontal of less than the angle of repose of loose coal and char for the particular coal seam, and the production well is positioned at an angle with respect to horizontal of greater than the angle of repose, but less than 90°. Each cavity in the operation can be individually valved to injection and production pipelines where a number of cavities are used in one coal seam. An oxygen-containing gas mixture is injected into the seam through the injection well and combustion products removed from the production well. An excess of oxygen-containing gas such as air or oxygen, or a mixture thereof, steam and oxygen, or carbon dioxide and oxygen is introduced to form a highly volatile and combustible combination within the coal deposit. This is ignited by electrical means or by the introduction of pyrophoric mixtures. In U.S. Pat. No. 4,662,443, Gash et al describe use of an air-blown underground coal gasification plant to produce low-BTU gas and an oxygen-blown plant for the production of product gas from which synthetic natural gas may be produced.
U.S. Pat. No. 4,422,505 to Collins describes a method for gasifying subterranean coal deposits by positioning a cased injection well to extend from the surface into the coal deposit with the injection well extending horizontally through the lower portion of the coal deposit with the horizontal portion of the well being cased with a perforated casing; positioning an injection tubing within the injection well; positioning a production well to extend from the surface to a point near the lower end of the injection well; igniting the coal deposit; gasifying a portion of the coal deposit between the bottom of the production well and the lower end of the injection tubing well by injecting a free-oxygen containing gas into the coal deposit through the injection tubing and recovering product gases through the production well and thereafter gasifying a second portion of the coal deposit by withdrawing the injection tubing a selected distance and thereafter injecting free-oxygen containing gas into the coal deposit and recovering product gases from the production well. By the process of this invention, the use of vertical gas injection wells is eliminated. The invention purports to utilize no coolant. Desirably, a high temperature alloy (e.g., stainless steel) injection nozzle is positioned on the lower end of the injection tubing.
Little progress has been made in processes for in situ gasification of coal in the past two decades primarily due to a lack of economic incentives. There remains the risk of potential environmental contamination of near-surface potable aquifers, due to working within proximity to the surface. There remain challenging technical problems such as the inability to adequately control the process. One particular technical problem is the ability to achieve high reaction temperatures at low cavity pressures, and thus the production of product gas of desired quality and quantity.
Accordingly, there remains a need for a safe and effective system and method for recovering energy from deeper carbonaceous deposits. In certain embodiments, the system and method should reduce the number of injection and/or production wells, thus minimizing surface disturbance, product leakage, and other negative environmental impact, should also allow for much deeper extraction of carbonaceous deposits, and/or should allow for higher reaction temperatures and pressures, and the control of these and other process parameters.